The removal of halogenated organic compounds from aqueous environmental sources including halogenated aromatic compounds such as chlorophenols and chlorobenzenes has posed serious problems. Typically, the halogenated compounds have been disposed of by separating, such as by steam stripping, using a microporous hollow-fiber membrane, or carbon adsorption, the contaminants from their aqueous environment and then subjecting the resulting concentrated levels of contaminants to incineration. However, the combustion of halogenated aromatics may result in the production of highly toxic by-products such as dioxins. Thus, incineration can itself become an environmentally unsafe practice and its use for the disposal of halogenated phenols is problematical.
Industry has therefore looked to alternative techniques for the destruction of halogenated aromatics found in the environment. Among the techniques which have been studied are biological treatment and chemical dehalogenation.
Chemical dehalogenation methods have been developed as an alternative to incineration and land disposal because they convert the halogenated organic compounds to less toxic non-halogenated compounds, in the case of halogenated phenols to phenol itself. One such process employs a sodium naphthalene reagent to form sodium chloride and an inert sludge. While the sludge can be safely incinerated, the process is complicated by requiring an air-free reaction vessel which limits its application for on-site treatment of contaminated environmental sources. In another approach, a dechlorination reagent is formed by reacting an alkali metal with polyethylene glycol in the presence of heat and oxygen.
The above-mentioned processes, which involve the oxidative dechlorination of halogenated organic compounds, are generally highly sensitive to water. Such processes require a separation step to remove the halogenated compounds from the aqueous environment before they can be treated. In addition, elevated temperatures are often required to carry out the reaction [See S. Tabaei et al., "Dehalogenation of Organic Compounds" Tetra. Let. 32(24) pp. 2727-30 (September 1991); M. Uhlir et al., "Recovery of Biphenyl by Catalytic Hydrogenolysis of Chlorinated Biphenyl" Chem. Abstr. 114 (23): 228496Z; and processes referred to in N. Surprenant et al., "Halogenated Organic Containing Wastes" pp. 224-231 Noyes Data Corp. (1988)]. Accordingly, these processes have not been widely accepted for the decontamination of environmental sites.
There has been developed a reductive process for the dehalogenation of halogenated organic compounds. J. F. A. Kitchen, U.S. Pat. No. 4,144,152 discloses a process for the treatment of halogenated organic compounds with UV radiation and hydrogen in the absence of an oxidizing agent. While this process may be conducted in an aqueous environment, the requirement of a UV radiation reactor has made light activated reduction of chemicals (LARC) processes of the type disclosed in U.S. Pat. No. 4,144,152 of limited commercial value. Although numerous references to hydrodechlorination using a palladium on carbon catalyst have been reported in the technical literature [Rylander, P. N. "Catalytic Hydrogenation over Platinum Metals," Academic Press, New York, 1967, pp. 405-431] no comprehensive study on the complete dechlorination of chlorophenols or chlorobenzenes has been reported.
There is therefore a need for processes in which halogenated organic compounds can be removed directly from aqueous contaminated environmental sources in a safe and cost effective manner. Such processes should be able to be conducted under mild reaction conditions and be effective in treating contaminated sources having both very low and very high concentrations of contaminants as is likely to be found in a variety of waste streams.